This application claims priority from Korean Patent Application No. 10-2004-0004984, filed on Jan. 27, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an edge type luminous backlight unit and a method of manufacturing a diffuser employed in the backlight unit, and more particularly, to a backlight unit designed to compensate for a difference in distribution of light output on an output surface of a light guide panel, and a method of manufacturing a diffuser employed in the backlight unit.
2. Description of the Related Art
Typically, a liquid crystal display (LCD) is a flat panel display and needs a separate light source to produce an image since the LCD itself does not emit light. To this end, a backlight is located behind the LCD and emits light. Backlight units are classified into direct light type units and edge type units according to the position of a light source arranged.
A general edge type backlight unit transfers light irradiated from a light source located on an edge of a light guide panel (LGP) to an LCD panel via the LGP. As the light source for the edge type backlight unit, a linear light source such as a cold cathode fluorescent lamp (CCFL), or a point light source such as a light emitting diode (LED) is employed.
Referring to FIGS. 1 and 2, a conventional edge type backlight is provided with three LEDs 1, which are point light sources disposed along one edge 11 of an LGP 10 and emit light toward the edge 11. The LGP 10 converts a light incident from the LEDs 1 into a surface light, and outputs the converted surface light in a vertical direction. For this operation, the backlight has a holographic pattern 15 at the bottom of the LGP 10, which allows light emitted by the LEDs 1 to be directed onto a light emission surface 12 and a diffuser 17 on the light-emission surface 12, which diffuses the emitted surface light from the light-emission surface 12.
The holographic pattern 15 is a diffractive grating structure, and converts incident light into surface light and diffracts the converted surface light onto the light-emission surface 12.
As the range of incident azimuth angles of light being incident on the holographic pattern 15 decreases, uniformity of brightness over the light-emission surface 12 increases. Uneven brightness over the light-emission surface 12 makes a screen appear motted. In a narrow range of about 1 cm or so, a brightness variation of about 0.9 is detected as a stain on the screen. However, a slow variation in brightness of about 0.8 between the screen's center and comers may not be detected as a stain. Hence, a brightness uniformity of at least 0.8 is required to prevent a stain on the screen. To achieve a better quality image, a brightness uniformity of 0.9 or more is required.
Referring to FIG. 3, the light-emission surface 12 is divided into three regions of a light input region 12a, a central region 12b and a large light region 12c, which are disposed in a direction away from the edge 11. At this point, when reviewing distributions of output lights in each of the three regions 12a, 12b, and 12c, the central region 12b has a wider distribution of the output light than that of the light input region 12a, and the large light region 12c has a wider distribution of the output light than that of the central region 12b. 
In the meanwhile, the diffuser 17 diffuses the surface light emitting from the light-emission surface 12 to obtain more uniform light, and has the same diffusion angle distribution. Accordingly, a distribution of the light emitting through the diffuser 17 is the same as that of the surface light emitting through the light-emission surface 12.
FIG. 4 is a graph illustrating brightness of lights emitting from the respective regions of FIG. 3. In FIG. 4, a vertical axis represents a front brightness and a horizontal axis represents a light emitting angle as a forward half maximum (FWHM) angle. Three curves C1 through C3 represent brightnesses of the light input region 12a, the central region 12b and the large light region 12c, respectively.
From the graph of FIG. 4, it is understood that the brightness of the light input region 12a is greater than that of the central region 12b. Also, the light input region 12a has a FWHM angle of 20□/20□ while the central region 12b and the large light region 12c have a FWHM angle of 20°/35°. In the representation of the FWHM angle, the values before and after the slash “/” denote FWHM angles in X and Y directions in FIG. 3, respectively.
Thus, such a difference in the brightness on each region 12a, 12b, or 12c arises because the range of azimuth angles of light incident on the holographic pattern 15 is narrower in the light incident region 12a than in the central region 12b and the large light region 12c. That is, since lights having various incident azimuth angles are incident onto the holographic pattern 15 after being reflected several times as shown in FIG. 2, such a difference is shown. The non-uniformity of the brightness is progressively more severe as the incident azimuth angle of the light emitting from the LEDs 1 and then incident onto the LGP 10 increases.
Accordingly, since the conventional backlight unit has a narrow output light distribution in the light incident region 12a and a wide output light distribution in the large light region 12c, non-uniformity and loss of the output light are caused.